Today, the JANSEN powerwave coax from Jansen is available in GHEtool Cloud. In this article, we shed light on the mathematical model behind this coaxial probe, with a focus on the fluid dynamic behaviour of the system.
JANSEN powerwave coax
The JANSEN powerwave coax is a coaxial geothermal heat exchanger developed by the Swiss company Jansen. It is a coaxial pipe with an outer diameter of 63 mm and an inner pipe of 32 mm, both with a wall thickness of 2.9 mm. What makes this pipe unique is the corrugated outer surface, which both increases the heat transfer area to the grout and changes the fluid behaviour inside the probe.
This corrugated outer surface changes the fluid behaviour and hence also the turbulence inside the pipe’s annulus. This changes both the heat transfer from the annulus to the grout and the heat transfer from the annulus region to the inner pipe. In order to quantify this heat transfer, the fluid behaviour in this probe first has to be quantified. Just like with the TurboCollector and the MuoviELLIPSE, which also altered the internal fluid behaviour in their own way, this can be quantified by relying on computational fluid dynamics, and more specifically direct numerical simulation.
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The model development of the JANSEN powerwave coax was presented at Der Geothermiekongress 2026 in Potsdam (Germany). This article is a popular-scientific description of this scientific paper. For more detailed information about the model used, the reader is referred to the actual paper, referenced below.
!Note
Besides the powerwave coax, Jansen also makes this corrugated pipe in a single or double U-configuration. Due to the significant difference in fluid behaviour (the single and double U are, of course, non-coaxial), the fluid behaviour inside these probes is again different and requires a study of its own. The other powerwaves will be discussed in a later article.
Model development
In this section, the model development of the JANSEN powerwave coax is discussed. First, the fluid simulations are discussed, followed by the correlations for the Nusselt number and friction factor.
Fluid simulations
Computational Fluid Dynamics (or CFD for short) is one of the most important fields in engineering today. It is used to simulate fluid behaviour in chemical plants, optimise the shape of aircraft wings to maximise lift, assess the aerodynamic performance of vehicles, predict wind turbine output, and much more.
Although different turbulence models exist, ranging from simplified and faster models like RANS or LES, the more accurate and computationally intensive Direct Numerical Simulation (or DNS) method is used in this study. With this approach, the turbulence gets calculated down to the smallest temporal and spatial scales, so both the thermal and hydraulic behaviour can be accurately studied. This is necessary in order to find the actual onset of turbulence in the powerwave coax, as well as the heat transfer in all different fluid regimes.
In the figure below, the fluid results are shown for different Reynolds numbers, ranging from 600 to 2000.
When looking at the different simulations above, it is clear that the case with a Reynolds number of 600 is still completely laminar, which can be seen by the clear distinction between the different colours that indicate the fluid temperature. Here, all layers are clearly moving next to each other and there is no mixing whatsoever.
When the flow rate is increased until a Reynolds number of 800 to 1000 is achieved, this clearly changes. At Re=800 the first vortices inside the wells are visible, but they mainly stay within the well itself, only causing minor disturbance in the overall flow inside the pipe’s annulus. At Re=1000 these disturbances start to propagate through the rest of the probe, reducing the laminar region significantly. Therefore, the onset of turbulence is said to be at Re=1000.
For higher Reynolds numbers, this mixing keeps growing, with the laminar region being gone at around Re=1400 and a clearly mixed flow for Reynolds numbers higher than 2000. However, for reasons of consistency, the fully developed turbulent flow is said to start at Re=4000.
Development of the correlations
Based on the fluid simulations above, the correlations for the Nusselt number (used for the convective heat transfer) and the friction factor (used for the pressure drop) can be derived. In contrast to the correlations developed for the TurboCollector and the MuoviELLIPSE, the powerwave coax requires two different Nusselt number correlations. This is because the fluid in the annulus exchanges heat both with the grout via the corrugated outer pipe wall and with the inner pipe. Both correlations are shown below.
When looking at the situation at the outer wall of the annulus, it is clear that the Nusselt number increases significantly at around Re=1000, which was exactly the onset of the turbulence region. For the inner pipe, this transition starts somewhat later and is less pronounced. Looking at the fluid simulations above, it is also clear that the strongest (and earliest) vortices happen in the wells rather than close to the inner pipe wall, which in turn is beneficial for the convective heat transfer.
!Note
Note that, for the Nusselt number, multiple simulations were performed for different Prandtl numbers, indicated by the red and blue colours. A detailed explanation of the Prandtl number is beyond the scope of this article, but for geothermal applications it typically ranges from approximately 20 (blue) to 75 (red), depending on the fluid properties, such as the type of antifreeze used, and on the fluid temperature.
For the friction factor, the correlation is shown above. Here, the friction factor decreases quite quickly in the laminar regime and stays quite constant in the transitional and turbulent regions.
Using the equations for the Nusselt number and the friction factor, the effective borehole thermal resistance and pressure drop graphs can be created. This will be discussed in the next section.
Simulation results
In this section, the effective borehole thermal resistance of the JANSEN powerwave coax will be discussed first, followed by the pressure drop.
Effective borehole thermal resistance
The graph below shows the effective borehole thermal resistance for the JANSEN powerwave coax, as well as for a regular coaxial pipe (with the same inner pipe of 32 mm and outer diameter of 63 mm) and a double DN32 probe for comparison. The results were obtained for a borehole diameter of 150 mm (unless stated otherwise) and a fluid consisting of 25 v/v% monopropylene glycol (MPG) for all cases. A borehole depth of 100 m was assumed, together with a grout thermal conductivity of 1.5 W/(mK).
In the graph above, it is clear that the powerwave coax outperforms the regular coaxial pipe for all flow rates. At around 0.7 l/s, the fluid in the annulus of the regular coaxial pipe also becomes turbulent, enhancing the heat transfer significantly. That is why, at larger flow rates, the regular coaxial pipe converges to the solution of the powerwave coax, which is already turbulent from around 0.2 l/s.
The small difference that is visible at around 0.2 l/s is where the inner DN32 pipe becomes turbulent. This increases the borehole resistance slightly since the thermal short-circuiting between the inner and outer pipe is now smaller.
Since a coaxial pipe is assumed to be at the centre, for the case of a 150 mm borehole, this implies that there is more than 40 mm of grout on all sides of the pipe, insulating the pipe from the wall. Therefore, when the powerwave coax is installed in a smaller borehole of 110 mm, the borehole resistance drops significantly, even outperforming the double U probe over a range of 0.2-0.4 l/s.
In the graph above, the borehole resistance is shown for a constant flow rate but for different drilling depths. It is clear that the effective borehole resistance increases with increasing depth, but significantly more for the coaxial probes than for the double U. This is because the thermal short-circuiting between the inlet and the outlet fluid is higher for a coaxial pipe (where the fluid is just separated by a single pipe wall) than for a U-tube, where there is grout and two layers of pipe wall between them.
This larger heat transfer between the entering and returning fluid increases the overall effective borehole thermal resistance and becomes more pronounced when the boreholes are deeper. This is true for any coaxial pipe.
Therefore, any coaxial pipe benefits from shallower depths as well as smaller drilling diameters. Here, it is clear that for a smaller drilling diameter, the JANSEN powerwave coax can outperform the double U up to a drilling depth of 120 m.
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For more information about the depth dependence of the effective borehole thermal resistance as well as the local and internal resistance, the reader is referred to our course, Part 2.2.
Pressure drop
The graph for the pressure drop is shown below, again for a borehole depth of 100 m and 25 v/v% MPG at 5°C. Since the borehole diameter is of no importance for the pressure drop, only one powerwave probe is shown below.
It is clear that, once again, the price one has to pay for the better heat transfer and enhanced turbulence is an increased pressure drop. Of all the pipes in the comparison, the JANSEN powerwave coax has the highest pressure drop, due to the turbulence that already starts in the annulus at Re=1000. The regular coaxial pipe needs a much higher flow rate to transition to turbulence and therefore also has the lowest pressure drop.
This, together with the story about the borehole resistance, shows that the JANSEN powerwave already gives a good borehole resistance at very low flow rates, where the pressure drop is also reasonably low. Since a coaxial pipe also benefits from shallower drilling depths, the pressure drop is automatically somewhat lower.
JANSEN powerwave coax in GHEtool
As of today, the JANSEN powerwave coax is available in GHEtool Cloud.
Conclusion
This article introduced the JANSEN powerwave coax in GHEtool and discussed its thermo-hydraulic behaviour. Based on a direct numerical fluid dynamic simulation, it was shown that the turbulence in the annular region of the pipe already starts at a Reynolds number of 1000.
Looking at the effective borehole thermal resistance, the JANSEN powerwave coax outperformed the traditional coaxial pipe for all flow rates. It was discussed that any coaxial pipe benefits from a smaller borehole diameter as well as a shallower drilling depth. When the borehole diameter was reduced to 110 mm, the powerwave coax also outperformed the double U DN32 case.
The downside of higher turbulence and better heat transfer is again a higher pressure drop. Therefore, this probe is mostly suited for shallower depths, where the flow rate can be low (due to the enhanced turbulence), thereby minimising the required pump energy.
References
- Watch our video explanation over on our YouTube page by clicking here.
- Peere, W., Hidman, N., Hofstetter, R. (2027). Development of a thermohydraulic model for the JANSEN powerwave coax with direct numerical simulation and its impact on the thermal borehole resistance. In proceedings of Der Geothermiekongress (DGK) 2026, 20-22 October 2026, Potsdam, Germany.
- Hidman, N., Almgren, D., Johansson, K., Nilsson, E. (2026). How internal fins enhance the thermohydraulic performance of geothermal pipes: A direct numerical simulation study. International Journal of Heat and Mass Transfer, 256(3), 128114